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Researchers in China have developed a water-based invisible ink that can be used to encrypt secret or sensitive information that’s printed on paper. The method is a low-cost and environmentally friendly alternative to fluorescent inks currently used for this purpose today, researchers said. (Image source: Nanjing University of Posts and Telecommunications)

Researchers from the Institute of Advanced Materials at the Nanjing University of Posts and Telecommunications in China have developed a simple and secure printing technology in the form of a water-based invisible ink that can be used to encrypt secret or sensitive information.

The rather low-tech method is basically a rewriteable paper coating that can be printed by a water-jet printer on a manganese-complex-coated paper.

The writing produced by the technique is invisible to the naked eye. However, a 254-nanometer UV light shined on the paper reveals the message. Moreover, the paper can be recycled and reused for up to another 30 rounds of printing after heating the paper with a blow dryer for 15 to 30 seconds, which erases the message.

The paper encryption method is not just low-tech, but also a low-cost and environmentally-friendly alternative to the current use of fluorescent security inks for protecting paper-based information, said Qiang Zhao, one of the researchers on the project and an associate professor of applied physics and computational mathematics at the institute.

“The rewritable feature significantly reduces the cost,” Zhao said in a press statement. Indeed, the cost per print is estimated to be RMB0.014 in Chinese yuan, or $0.002 US, he said.

"Most fluorescent security inks on the market used to record confidential information are environmentally unfriendly and cannot be erased,” Zhao added. “The paper is only a disposable recording medium."

Zhao’s team developed the new method based on previous work researchers had done to regulate organic materials' photoluminescence properties by modifying the molecular structure, he said. The team recently discovered that the material could change its optical or electrical properties through external stimulus alone.

“We developed a rewritable security printing method by utilizing the photoluminescence responses of manganese complex to water,” he said in the press statement.

Enhancing Safety and Security

Researchers view their water-jet printing method as a cost-efficient way to encrypt printed materials. But they said it does have one drawback – the short-wavelength UV light used to trigger the water-jet security printing can be harmful to humans.

Because of this, Zhao said the team is focused on developing humidity-sensitive manganese complexes that can be excited by visible or near-infrared light, which is much safer for humans.

"Our work is to provide a practical printing method,” Zhao said in the press statement. “Thus, we need to make sure that it's non-toxic or has low harm to the human body.”

The team also is developing a high-level security printing to complement the low-tech, water-jet technique to protect information from general decryption methods, researchers said.

In this technique, researchers coat the paper with phosphine ligands, which can grab on to manganese in the manganese-halide salt solution ink to create manganese complex, they said.

This renders the recorded information invisible under both ambient light and UV light; however, a photoluminescence lifetime imaging (PLIM) technique can reveal the encrypted data in different colors of red, yellow, green, and blue, depending on the emission lifetime, Zhao said.

"The dynamic manipulation of the emission lifetime has been achieved for the first time by utilizing the reversible ionic interactions of manganese complexes," he said in the release.

The team envisions that their techniques can be used widely in both the public and private sector to protect the security of information, especially in the economic and military fields, Zhao added.

Elizabeth Montalbano is a freelance writer who has written about technology and culture for more than 20 years. She has lived and worked as a professional journalist in Phoenix, San Francisco and New York City. In her free time she enjoys surfing, traveling, music, yoga and cooking. She currently resides in a village on the southwest coast of Portugal.

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